GROWTH OF THE SHELL. 69 



The typical structure is that of an enlarging conical tube, 

 winding obliquely from left to rights by reason, probably, of 

 some peculiar tendency in the vital organs of the animal. 

 The axis upon which the tube winds, is called the axial 

 pillar, or columella ; every turn round this axis is termed a 

 w/iorl ; and when the columella is hollow, it is said to be 

 umbilicated. Modifications exist in different species; and 

 while some convolutions are extremely simple, others are 

 elongated or depressed. Of this, the shell of the Caracolla 

 parmula, which is so much flattened in its spiral tube as 

 to present the form of a slightly convex lens, offers a fami- 

 liar instance. The form of such as pertain to the second 

 division varies, from the deeply depressed cone of the TJm- 

 brella, to the extreme conical elevation of Dentalmm. In 

 one genus. Chiton^ the shell consists of eight distinct pieces, 

 moving one upon the other within a cartilaginous frame, 

 and bearing no small resemblance to plate armour. In the 

 Aplysia, the shell is merely represented by a concealed horny 

 plate, deposited by an internal fold of the mantle. There 

 are likewise shell-less members of the same community, 

 called Nudihranchiata, or Naked-gilled Mollusks, which 

 present beautiful varieties of tufted and ramified structures 

 along their backs. They afford no ornaments for the cabi- 



